Laser-induced incandescence techniques for soot measurements: capabilities, opportunities, and challenges

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Laser-induced incandescence techniques for soot measurements: capabilities, opportunities, and challenges

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Laser-Induced Incandescence Techniques

for Soot Measurements: Capabilities,

O

t

iti

d Ch ll

Opportunities and Challenges

Fengshan Liu Fengshan Liu

Institute for Chemical Process and Environmental Technology National Research Council Canada

Ottawa ON Canada K1A 0R6 Ottawa, ON, Canada K1A 0R6

中国工程热物理学会 2009 年燃烧学学术年会 31 October 2009, Hefei, China

(3)

Outline

• Background

• A brief history of LII

A brief history of LII

• Introduction of LII: principle, setup, issues

• LII Theory

• LII applications in combustion: capabilities and

opportunities

• Remaining challenges

• Conclusions

(4)

Background

Desired and undesired aspects of soot

Improve efficiency of combustion technology

Desired and undesired aspects of soot

Engineered nanoparticle Engineered nanoparticle

synthesis

Assess the ecological and health impacts of Assess the ecological and health impacts of

(5)

Background

Emission of soot from various combustion devices • Emission of soot from various combustion devices

(automobiles, furnaces, gas turbines) into the environment is detrimental to human health and a major contributor to climate change

climate change

• Our understanding of soot formation is still quite limited

• Modern and future generation combustion devices

(engines, combustors) produce less soot, which requires highly sensitive diagnostic techniquesg y g q

• Morphological information of soot (fractal properties, primary particle size, aggregate size distribution) is

i d t i f th f d t l i i ht i t t required to gain further fundamental insights into soot formation and to assess their health impact

(6)

Background

• It is desirable to develop real-time, in-situ, and sensitive measurement techniques to meet these challenges

• Conventional techniques (e.g., light extinction, laser scattering) do not meet these requirements

L I d d I d (LII) i i i

• Laser-Induced Incandescence (LII) is a promising

candidate technique and has been rapidly developed into a powerful diagnostic tool for soot and other nanoparticle

h t i ti characterization

(7)

Outline

• Background

• A brief history of LII

A brief history of LII

• Introduction of LII: principle, setup, issues

• LII Theory

• LII applications in combustion: capabilities and

opportunities

• Remaining challenges

• Conclusions

(8)

History of LII

• The first reported study related to LII was perhaps made by • The first reported study related to LII was perhaps made by

Gelbwachs and Brinbaum in 1973 (Applied Optics) (Aerospace Corp., California)

Th t d th h f b db d “fl f

• They reported the phenomenon of broadband “fluorescence of atmospheric aerosols” as an interfering signal to the molecular

fluorescence used for detecting gaseous pollutants (NO₂ in their case) using a cw argon ion laser (458 nm to 515 nm)g g ( )

• They suggested that “Fluorescence is a potentially useful means for aerosol identification and monitoring.”

• In 1974, Weeks and Duley (York University, Toronto) published the

very first paper (Aerosol-particle sizes from light emission during excitation by TEA CO₂ laser pulses) to use the concept of LII as a means to determine particle size

means to determine particle size

• These researchers are the pioneers of LII by recognizing that “the passage of intense laser radiation through a dusty atmosphere

lt i th t i i f i ibl li ht f ti l

results in the momentary emission of visible light from particles in the path of the beam.”

(9)

History of LII

B th t i i t di t i th f

• Because the two pioneering studies are not in the area of soot and flame, the LII community and literature paid no attention to these two important pieces of workp p

• Eckbreth in 1977 also observed the phenomenon of LII

in a sooting laminar propane diffusion flame (JAP: Effects in a sooting laminar propane diffusion flame (JAP: Effects of laser-modulated particulate incandescence on Raman scattering diagnostics)

• The particle laser energy absorption and the soot particle evaporation processes were analyzed

evaporation processes were analyzed

• Eckbreth also detected LII signals at two wavelengths

t it th k t ti l t t

(10)

History of LII

M lt i 1984 bli h d th fi t u rnal s 20 25

• Melton in 1984 published the first

paper to explicitly explore LII as a diagnostic for soot measurement

(S t di ti b d pers in core jo

u

15

(Soot diagnostics based on laser heating)

• Melton also formulated the first LII umber of LII pa

p

5 10

• Melton also formulated the first LII model, which remains the

backbone of subsequent models

Year

1970 1980 1990 2000 2010

N

u

0

• Eckbreth’s work and Melton’s were regarded

as the birth of modern LII techniques for

measurement of soot and other nano-sized particles

• Research attention was paid to LII starting from the early 90’

(11)

History of LII

• Early research and applications were in Europe, US, and Canada

• It has gained increased attention in Japan, South Korea, and China

• The driving force for the popularity of LII is the

concerns of soot emissions from various combustion devices and the need to advance our fundamental

devices and the need to advance our fundamental understanding of soot formation

(12)

History of LII

• Three international LII Workshops have been held in theThree international LII Workshops have been held in the

last few years (www.liiscience.org)

2005 2006 2008

4th _{International Workshop and Meeting on Laser-Induced Incandescence} 19-20 April 2010, Lake Como, Italy

(13)

Outline

• Background

• A brief history of LII

A brief history of LII

• Introduction of LII: principle, setup, issues

• LII Theory

• LII applications in combustion: capabilities and

opportunities

• Remaining challenges

• Conclusions

(14)

Principle of LII

• LII experiment: FilterFilter

Photo-detector Photo-detector

• LII experiment:

– pulsed laser beam (ns)

– rapid heating of soot to about 4000 K

Pulsed Laser Beam Pulsed Laser Beam

Collection Lens Collection Lens

4000 K

– soot radiates incandescence while cooling to ambient

temperature Particulates Source Particulates Source – incandescence signal is collected to determine soot concentration, specific surface

area and primary particle 0 0

0.0 0.0

T

area, and primary particle diameter 0.0 0.0 0.0 LII signal 0.0 0.0 Laser pulse (30 ns) f v ∝ LII max Time, ns 0 100 200 300 400 0 0.0 (30 ns)

(15)

LII Set-ups

• LII components:LII components: Laser source particle source (flame orLaser source, particle source (flame or

generator),signal collection system

Flow out to GaAs PMT Compact PMT’s 3-Way Fiber Splitter Transient Digitizer NRC Set-up B

Receiver Lenses _{Fiber Input} Filter Receiver Fiber Optic Cable Splitter Set-up Compact Nd:YAG Laser Half Wave Plate Beam Dump Iris Test Cell Nd:YAG Laser Polarizer Cylindrical Focusing Lens

Flow in from the Dilution Tunnel

(16)

LII Set-ups

(17)

NRC LII Apparatus

electrical and water to/from power supply

detection package (collection control and signal lines

to/from photodetectors

detection package (collection optics, beam separation optics, and photodetectors)

beam dump Nd:YAG laser head (1064 nm) ½ wave platep ½ wave plate thin film polarizer

35º

detector track vertical slit

measurement location

(18)

Commercial LII Devices

http://www lavision de/techniques/lii php La Vision http://www.lavision.de/techniques/lii.php http://www.artium.com Applications:

- in-situ and real time characterization of soot

soot formation in a

Common Rail 2l diesel Artium

emission in diesel and direct injection spark ignition engines, gasturbines, flames and various kinds of metal or ceramics

(19)

Issues for LII System

1e+13 • Excitation wavelength (1064 nm , W m -3 st e r -1 1e+11 1e+12 3500 K 4000 K 4500 K Excitation wavelength (1064 nm or 532 nm)

• Laser fluence choice: high or low

Intensit y , 1e+8 1e+9 1e+10 2500 K 3000 K

Solid lines: blackbody Dashed lines: particle

• Detection issues: wavelength, gated or time-resolved, gated width and timing • Point LII (PMT) or 2D LII (ICCD)

0.0

Wavelength, nm

200 300 400 500 600 700 800 9001000 1e+8

• Point LII (PMT) or 2D LII (ICCD) • Calibration: (1) conventional approach f = C S 0.0 0.0 T f_v = C S_LII

(often through comparison with laser extinction)

0.0 0.0

LII signal

(2) absolute intensity approach (detect absolute LII intensity

and particle temperature)

( ti l t t i 0.0 0.0 Laser pulse (30 ) (particle temperature is

measured through two-color

LII detection) 0 100 _{Time, ns}200 300 400

0 0.0

(20)

Features of LII

• Relatively easy to implement • Relatively easy to implement • Non-intrusive (?)

• Very high temporal resolution (turbulent flames, engineVery high temporal resolution (turbulent flames, engine combustion)

• Very high dynamic range (ppb up to ppm)

• Good spatial resolution for point measurement

• 2D setup (soot imaging in laminar and turbulent flames, engine cylinder)

cylinder)

• Provides soot volume fraction and primary particle size • However, it requires sophisticated theory to quantitatively

understand the processes to determine primary particle size (distribution)

(21)

Outline

• Background

• A brief history of LII

A brief history of LII

• Introduction of LII: principle, setup, issues

• LII Theory

• LII applications in combustion: capabilities and

opportunities

• Remaining challenges

• Conclusions

(22)

Theory of LII

LII th

• LII theory

– Numerical model of nanoscale (temporal and spatial) heat and mass transfer to and from the particlesass a s e o a d o e pa c es

– LII theory is essential to interpret the detected signals,

especially for time-resolved LII to obtain particle size information

Radiat ion

Absorpt ion

Heat conduct ion Int ernal

Heat conduct ion energy

(23)

Structure of soot

• Need to know the structure of soot to establish LII theory • Need to know the structure of soot to establish LII theory • How soot particles look like?

form fractal aggregates - form fractal aggregates

- not isolated spherical particles • Primary particles are more or

less uniform in size

Megaridis and Dobbins, Combust. Sci. Tech.

(24)

Conventional LII Model:

single particle based

• Assumptions: neglect the effect of aggregation on particle laser • Assumptions: neglect the effect of aggregation on particle laser

absorption and particle cooling

Bridging primary Single primary soot particle: 10-60 nm Bridging primary soot particles in an aggregate Just-touching primary

(25)

LII Theory

• LII model: schematic length scales for soot in flames

1 atm

T = 1700 K MFP: 600 nm

(26)

LII Theory

• Time and length scales forTime and length scales for 9e+11 laser absorption e r, W /m 2 5e+11 6e+11 7e+11 8e+11

Primary soot particle: 30 nm Laser wavelength: 1064 or L aser pow e 2e+11 3e+11 4e+11 5e+11 7 ns FWHM Laser wavelength: 1064 or 532 nm Heat diffusion time:

2

d

time, s

0.0 5.0e-9 1.0e-8 1.5e-8 2.0e-8 2.5e-8 3.0e-8 3.5e-8

L 0 1e+11 / p d s p d k c

τ

ρ

= , which is in sub-nanosecond Particle is isothermal / p d

π λ is a small value (~0.1), i.e. Rayleigh regime

(27)

LII Theory

• Energy Equation for a Single Primary Soot Particle

(

)

(

)

2 3 0 2 d _a _v d s s a rad k T - T D _H D T M c C q -

π

+ + q

π

_ρ

₌ Δ

• Energy Equation for a Single Primary Soot Particle

(

)

d d s s a rad MFP v q q 6

ρ

t D + G

λ

M t

I

II

III

IV

V

I

II

III

IV

V

I particle internal energy variation rate II laser heating 2 3 a m D C π E λ = 8 f g

III heat transfer to surrounding gas IV soot evaporation V di ti h t l 8 ( 1) f G = _α _γ ₊ (9 5) / 4 f = γ − V radiative heat loss

• Other physical processes neglected, such as photodesorption, annealing, and oxidation

Large uncertainty in E_m and α exists

(28)

LII Theory

• Mass Transfer Equation for Soot Particle

2 2

d

1

_v s v

M

dD

D

D N

ρ π

π

=

= −

• Mass Transfer Equation for Soot Particle

d

t

2 ρ

s

dt

v

N

_A

I

II

I

II

I particle mass loss rate II vaporization rate P N RT 2 v A v v P N RT N RT M

β

π

= _{molecular flux}

(29)

LII Theory

• Radiation loss term

5 4 3 3 5

8 ( )

k

t

q

π

D E m

T

dt

∞

=

_{4 3}

∫

0

8 ( )

1

rad _t

q

D E m

T

dt

h c

e

π

=

−

∫

• LII signal intensity

2 3

8 ( )

(

)

c hE m

D

hc

S

_LII

π

₆

( )

D

3

exp(

)

S

kT

λ

∝

−

(30)

Some numerical

results

• Relative importance of heating and cooling termsRelative importance of heating and cooling terms

Tophat spatial profile at 6 mJ, d_p = 32 nm, E(m) = 0.261.

/s 1 +13 ion, J o ul e / 1e+11 1e+12 1e+13 Laser heating Heat conduction Soot evaporation rgy e qua ti 1e+9 1e+10 e Thermal radiation 8e+11 9e+11 532 nm n t h e en e r 1e+7 1e+8 p ow e r, W /m 2 4e+11 5e+11 6e+11 7e+11 8e+11 0.0 50.0 100.0 150.0 200.0 Te rm s i n 1e+5 1e+6

0.0 5.0e-9 1.0e-8 1.5e-8 2.0e-8 2.5e-8 3.0e-8 3.5e-8

Lase r p 0 1e+11 2e+11 3e+11 7 ns FWHM Time, ns time, s

(31)

Particle sizing

strategies

• Larger particles cool slower than smaller onesLarger particles cool slower than smaller ones:: Internal energy ∝ d_p3 Conduction cooling ∝ d_p2 3000 3250 3500 dp 40 nm 50 nm 60 • The time-resolved LII signal carries

information about the Te

m p era tur e, K 2000 2250 2500 2750 20 nm 30 nm 60 nm 70 nm particle size

• Particle temperature can Time, ns

0 500 1000 1500 1500 1750 2000 10 nm 20 nm p

be inferred from two-color or multi-color LII detection

7.0 7.5 8.0

dp = 40 nm

• The Temp decay rate is inversely

proportional to d_p ln( Te -T g ) 5 5 6.0 6.5 dp = 29 nm Time, ns 0 500 1000 1500 2000 5.0 5.5

(32)

Outline

• Background

• A brief history of LII

A brief history of LII

• Introduction of LII: principle, setup, issues

• LII Theory

• LII applications in combustion: capabilities and

opportunities

• Remaining challenges

• Conclusions

(33)

Capabilities of LII

• S tead y and flickerin g

laminar diffusion flames

yg

Shaddix and

(34)

Capabilities of LII

266 nm 1064 nm

• Turbulent jet diffusion

266 nm 1064 nm

Turbulent jet diffusion flames (ethylene)

7.6 to 9.5 cm above the burner

(35)

Capabilities of LII

• Laminar diffusion flames (d )Laminar diffusion flames (d_p)

Will et al., Optical Letters, 22, pp. 2342-2344, 1995.

e r distribu tion 0.06 0.08 Exp. data Present approach Error minimisation particle diamet e 0.04 0 10 20 30 40 50 60 70 80 P rim ary soot 0.00 0.02 d_p, nm 0 10 20 30 40 50 60 70 80

(36)

Capabilities of LII

• 2D Two-Color Time-Resolved LII (f_v and d_p)

Averaged 2D soot fraction distribution inside an engine

b i h b combustion chamber. From left to right: 7,9,11,13 CAD ATDC

Mean soot volume fraction and local mean diameter evolution for different crank angle

(37)

Summary of LII

Applications

• LII has become the preferred method for soot measurement in • LII has become the preferred method for soot measurement in

many areas of combustion:

- laminar flames (atmospheric and elevated pressures)

- shock tube

- droplet combustion

- in-cylinder engine combustion

- turbulent flames

- engine exhaust

- particulate concentration in environment (high sensitivity LII)

• LII has been applied to carbon black industry to do real-time monitor of particle size and to medical applications

• LII has also been used to other non-carbon refractory particles, such as metal oxides, synthesized in flames

(38)

Opportunities for LII

in Soot Studies

• Capabilities of LII: soot volume fraction primary particle size

• Capabilities of LII: soot volume fraction, primary particle size,

primary particle number density, the latter is unique to LII and is critical to understanding soot formation

• LII provides very high temporal resolution and very wide dynamic range

• LII can be combined with LIF and Laser Scattering to obtain further information on soot formation processes, such as relative spatial locations of PAHs and soot and soot

relative spatial locations of PAHs and soot and soot aggregation

• These capabilities make LII a powerful tool to advance

• These capabilities make LII a powerful tool to advance fundamental understanding of soot formation and

to investigate soot in turbulent flames and in engine combustion

(39)

Outline

• Background

• A brief history of LII

A brief history of LII

• Introduction of LII: principle, setup, issues

• LII Theory

• LII applications in combustion: capabilities and

opportunities

• Remaining challenges

(40)

How different LII models

compare?

(41)

How does LII model

perform in high-fluence?

perform in high fluence?

S h l t l C b ti d Fl 2000

(42)

Need to consider the

fractal structure of soot

z Three dimensional display of a z Three-dimensional display of a soot aggregate (129) D_a 0 9 1.0 α = 0.1 /N p πdp 2 0.7 0.8 0.9 Brasil et al. α = 0.37 present model πdef f 2 / 0.5 0.6 0.7 Koylu et al._2 Koylu et al._1 N_p 0 50 100 150 200 250 300 350 400 450 500 0.4 Filippov et al. (α = 1)

(43)

Need to consider the

fractal structure of soot

Is Rayleigh Debye Gans approximation acceptable for LII • Is Rayleigh-Debye–Gans approximation acceptable for LII

application? 1 20 ) 1.15 1.20 GMM: x_p = 0.177 GMM: x_p = 0.354 a gg /(N*C p abs ) 1.05 1.10 GMM: x_p = 0.0886 C abs a 0.95 1.00 Df RDG N 1 10 100 1000 0.90 k_f= 2.3, D_f= 1.78, N = k_f(R_g/a)Df N

(44)

Remaining Challenges

• How important is C and other non-carbon emissions to • How important is C₂ and other non-carbon emissions to

LII signal detection when 532 nm laser is used?

• Improvement over 2D two-color time-resolved LII forImprovement over 2D two color time resolved LII for volume fraction and particle size measurements

• Shot noiseShot noise

• Selection of laser fluence, detection wavelength(s)

• Is LII signal truly proportional to soot volume fraction?

S_LII ∝ d_p3+154nm/λ

(45)

Remaining Challenges

• Uncertainty in soot refractive index and thermal properties at ∼ 4000 K

• How important are other physical and chemical processes? (annealing, photodesorption, soot oxidation)

• How accurate is the soot sublimation model for a single particle?

f ff f

• How to account for the effect of particle aggregation on soot sublimation?

(46)

Outline

• Background

• A brief history of LII

A brief history of LII

• Introduction of LII: principle, setup, issues

• LII Theory

• LII applications in combustion: capabilities and

opportunities

• Remaining challenges

(47)

Conclusions

• LII offers many advantages over conventional techniques • LII offers many advantages over conventional techniques

and has evolved into a powerful diagnostic tool for soot measurement in combustion applications

• Its ability to infer primary soot particle size could potentially make significant contributions to advance our fundamental understanding of soot formation

understanding of soot formation

• The current single particle based LII model is reasonable

for inferring primary particle size in the low-fluence regime o e g p a y pa t c e s e t e o ue ce eg e

and under flame conditions

• Soot sublimation is still a poorly understood process and p y p hinders our quantitative understanding of LII in high-laser fluence regime

(48)

Conclusions

• Particle aggregation should be accounted for in LII theory • LII alone cannot provide sufficient information to p

determine soot morphology; Combined LII/LS is a

promising technique to obtain more complete information about morphologyp gy

• Many unanswered questions remain in the LII theory and practice to make LII more accurate and more reliable,

hi h ff h i

(49)